The present invention relates generally to the field of non-contact electrical measurement devices and methods and, more specifically, to the field of non-contact electrical probes, probe tips, and methods of using the same.
Recently, a number of systems and methods have been developed for making non-contact electrical measurements on semiconductor devices, integrated circuit chips, and the like. The system disclosed in U.S. Pat. No. 4,837,506 (hereinafter "the '506 patent"), which is incorporated herein by reference, is exemplary of such a non-contact measurement system.
The '506 patent discloses a system which utilizes a stimulation source of ultraviolet light and the "photoelectric effect" to make non-contact quantitative measurements on relatively small electrical devices such as printed circuit boards, semiconductor devices, integrated circuit chips and the like.
The system disclosed in the '506 patent comprises three major components, including an ultraviolet light source, an electron detector, and a sample chamber capable of being evacuated and accommodating the electron detector. The function of the system disclosed in the '506 patent may be summarized as follows. First, a sample (for example, an integrated circuit chip) is placed within the sample chamber on a sample stage. Next, the chamber is evacuated and a vacuum is established within the chamber. Those skilled in the art will appreciate that the vacuum established must be sufficient to allow electrons to easily travel between the sample and the electron detector. Once a sufficient vacuum is established within the chamber, the ultraviolet light source is activated, and a beam of ultraviolet (UV) light is focused and directed onto a measurement site located on the surface of the sample. When illuminated by the ultraviolet beam, the measurement site releases electrons, and the electrons, in turn, are collected by the electron detector which is disposed substantially adjacent the sample. The net charge flow between the sample measurement site and the electron detector and the voltage of the electron detector are measured and used to determine the voltage of the measurement site.
Those skilled in the art will appreciate that, because the sample is comprised of a "known" element (for example, aluminum), the "work function" of the sample is also known. The term "work function" refers to an amount of energy which is required to "free" an electron from the sample when the sample is illuminated by a beam of light having a given frequency. The energy E of a beam of light may be determined from the formula: EQU E=hV (1)
where, h is Plank's constant, and V is the frequency of the light beam. Further, because the energy of the light beam must equal or exceed the work function of the sample when electrons are released, and because Plank's constant and the work function of a given sample are known, the frequency of light (or its inverse, the wavelength) required to free electrons from a given sample may be readily calculated. For example, in a typical case of an integrated circuit having an aluminum conductive layer, the work function of the conductive layer is between 3.8 and 4.2 eV, and light having a wavelength in the ultraviolet range (225-275 nm) is required.
In light of the above, it will readily be appreciated that, in conventional non-contact measuring systems, such as that disclosed in the '506 patent, the composition of the sample determines the wavelength of light which must be utilized by the system to achieve electron release. Because of this fact and because many semiconductor devices comprise substances having work functions in the 4.0-5.0 eV range, it is often necessary to use light in the ultraviolet spectrum to effect electron release from a sample. This is somewhat disadvantageous because optics capable of accurately focusing photons of ultraviolet light on a measurement site are very expensive.